1. Field of the Invention
The present invention generally relates to a hydrodynamic gas bearing structure and, more specifically, to a hydrodynamic gas bearing structure used for a motor of a magnetic recording apparatus such as a hard disc, or of a magneto-optical recording apparatus.
2. Description of the Background Art
Recently, high rotational accuracy as well as high rotation speed have been required of a rotation driving part of a magnetic recording apparatus, for example. In order to rotate at a high speed a precision motor of which such high rotational speed and the high rotational accuracy are required, use of a hydrodynamic gas bearing in the rotation driving part has been proposed. In the rotational driving part using the hydrodynamic gas bearing, when a rotor rotates, air is forced at least into a gap between a radial type gas bearing body and the rotor. Accordingly, air pressure in the gap is increased, and the rotor rotates at high speed through the hydrodynamic gas bearing. Thus, by the use of the hydrodynamic gas bearing, it is expected that the rotational accuracy can be maintained even during high speed rotation.
Conventionally, various methods for controlling position of the rotor in a thrust direction has been proposed for this type of hydrodynamic gas bearing structure.
As a method of controlling the position of the rotor in the thrust direction, a method has been proposed in which two opposing thrust plates are provided above and below the rotor, whereby the position of the rotor is regulated from above and from below.
In another proposal, an attracting force between a yoke and a magnet of the motor is utilized to press the rotor to a pair of thrust plates, so as to control the position of the rotor in the thrust direction.
In an apparatus using such a hydrodynamic gas bearing as described above in the rotation driving part, a design allowing repetitive starting/stopping of rotation frequently is desired to reduce power consumption.
When rotation is started with an entire surface of the thrust plate provided for controlling the position in the thrust direction being in tight contact with the rotor, rotation torque of the motor necessary for starting increases, resulting in increased power consumption. In order to meet the large rotation torque, it becomes necessary to enlarge the size of a magnet coil of the motor. This does not meet the demand of reduced size of the apparatus.
In order to solve these problems, Japanese Patent Laying-Open No. 9-126229 proposes a method in which surface roughness of contact portions between the rotor and the radial bearing and between the rotor and the thrust bearing constituting the hydrodynamic gas bearing, is increased to enable smooth starting of rotation and to reduce time necessary for starting. Even when this method is adopted, however, the rotation torque necessary for starting rotation cannot sufficiently be made small.
Japanese Utility Model Laying-Open No. 5-73313 discloses a structure of a hydrodynamic gas bearing apparatus employing a pivot type bearing portion in place of the hydrodynamic bearing, as a bearing body in the thrust direction. According to this method, when the rotor is stationary, the rotor is supported by point contact, and therefore only an extremely small rotation torque is necessary to start rotation.
When the pivot type bearing portion is employed, however, the rotor is kept in contact with the bearing portion during rotation, and therefore the bearing portion wears because of high speed rotation. Even in an oil bearing apparatus employing a pivot type bearing portion, the bearing portion degrades as the temperature of oil increases, and oil scatters.
Therefore, this type of bearing apparatus is not suitable for high speed rotation, either.
In the hydrodynamic gas bearing apparatus utilizing the attracting force between the torque and the magnet of the motor to control position of the rotor in the thrust direction, sometimes it is the case that the symmetry thereof is unsatisfactory, resulting in a constant moment causing inclination of the rotor with respect to the bearing portion. This results in larger torque when starting rotation, and thus life of the hydrodynamic bearing becomes shorter.
It is possible to reduce torque for starting rotation, when a protruded portion is formed on an inner periphery of the thrust plate. In that case, a dynamic pressure generates as the number of rotation increases, generating a repulsive force between the protruded portion of the thrust plate and the rotor. When the height of the protruded portion is high, the generated dynamic pressure is small, and therefore, even when the rotor is rotated at a high speed, the protruded portion of the thrust plate is kept in contact with the rotor. This results in local friction heat at the contact portion, increasing temperature and causing wear at the contact portion.
Therefore, the height of the protruded portion should desirably be as small as possible. By forming a small protruded portion either on an end surface of the rotor or an inner surface of a housing opposing to the end surface, constituting the thrust hydrodynamic gas bearing, it is possible to start rotation of the rotor easily with a small driving force, as described in Japanese Patent Laying-Open No. 9-318900.
When the height of the protruded portion is reduced to such an extent that the height of the protruded portion is within the range of general unevenness existing on the surface of the thrust plate, the uneven protruded portions existing on the surface of the thrust plate would be in contact with the rotor. Therefore, if such a thrust plate is to be manufactured, it is necessary to process the surface of the inner periphery to be flat, except for the small protruded portion, which process undesirably increases manufacturing cost. Though it is possible to form the protruded portion as a separate member from the thrust plate, the number of manufacturing steps is increased, resulting in increase manufacturing cost.